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The mineralisation of soil organic sulphur (S) by soil micro-organisms constitutes a significant source of S for pasture plant growth. Cultivation and fertiliser applications affect microbial activity which in turn affects soil S mineralisation and the release of plant-available S. A field trial was conducted with cultivated and uncultivated subplots superimposed on the main plots of a long-term pasture trial in New Zealand receiving annual applications of superphosphate (0, 188, 376 kg ha-1) since 1952. The aim was to provide information on soil S mineralisation and availability as affected by cultivation and superphosphate applications under field conditions. Carrier-free 35SO4-S was applied to all plots in the field, allowed to pre-condition for two weeks before perennial ryegrass (Lolium perenne L.) seeds were sown to cultivated sub-plots, while in uncultivated subplots existing pasture was cut and removed and then allowed to re-grow. Five plant harvests followed by soil sampling each time were conducted over a period of one year. Herbage and soil samples were analysed for total S and 35S and different extractable soil S and 35S fractions (HI-reducible S, C-bonded S). Results obtained showed that cultivation and fertilisation significantly affected plant dry matter yield, soil S mineralisation and plant S uptake. These effects were affected by seasons. Plant S uptake was strongly related to soil S supply to plants and the plant S uptake provided a better measure of soil S availability to plants than changes in the extractable soil S fractions examined in the current study. [ABSTRACT FROM AUTHOR]

Although considerable progress has been made in relating extractable soil S to plant S availability, most of these studies determined the extractable soil S at the beginning of the experiment to use as an index of soil S status. This bears little or no relationship to the S taken up by plants during the entire growing season. The present study investigates the changes in extractable soil S with time and relates these to changes in plant S uptake. Six soils with different long-term fertiliser histories (0, 21, 42 kg of S as superphosphate ha-1 applied since 1952) and animal camping treatments (camp and non-camp) were used in two pot systems (with and without plants). Carrier-free 35SO4–S was added to the soils, to provide the information on the transformations of recently added S between the different extractable S forms in soils and whether these transformations could predict plant-available S. The soils were pre-conditioned and then transferred to the glasshouse, where one set of pots were planted with perennial ryegrass ( Lolium perenne L.) while the other set was left uncropped. Periodic plant harvests and soil samplings at four weekly intervals were conducted over a period of 20 weeks to determine plant S uptake and amounts of extractable soil S and 35S forms using five extractants. Same extractions of soil S and 35S were conducted for the initial soils. Results showed that HI-reducible and total soil S extracted by CaCl2, KH2PO4 and by KCl at 40 °C were utilised significantly by plants but not those extracted by NaHCO3 and NaOH extractants. However, after the 8th week, plants continued to take up S even though levels of S extracted from the soil by CaCl2, KH2PO4 and by KCl at 40 °C remained low and unchanged. These results suggest that soil S taken up by plants after the 8th week period originated directly from the mineralisation of soil organic S from S pools other than those present in the extractable soil S forms. Similar results were shown by 35S data, thereby confirming the complexity of determining plant S availability based on soil S extraction methods. [ABSTRACT FROM AUTHOR]